Shock wave lithotripsy is a non-invasive method for the treatment of renal calculi (kidney stones), biliary calculi (stones in the gallbladder or in the liver), and the like. In early systems for extracorporeal stone fragmentation, generally referred to as electrohydraulic shock wave lithotripsy, a probe (or electrode) containing two conical tips separated by a small distance (e.g., about 1 mm) is used for shock wave generation. When electric current is passed between the tips of the electrode, a spark is created that vaporizes the water or other surrounding fluids to create a shock wave. For example, U.S. Pat. No. 3,942,531 to Hoff et al. discloses the use of a spark gap discharge in water to generate a shock wave within an ellipsoidal reflector that collects and focuses the shock wave to fragment kidney stones inside the body. Such systems are generally effective in treatment, but the electrodes are prone to deterioration and thus need to be replaced periodically. The replacement of the electrodes is an added cost, an additional task to be performed during the course of a treatment, and an increase in the total amount of time necessary to complete the lithotripsy process.
By comparison, electromagnetic shock wave lithotripters use the rapid vibration of thin metallic membranes driven by electromagnetic forces to generate a shock wave rather than electric spark discharges. For instance, U.S. Pat. No. 4,655,220 to Hahn et al. discloses a device using a coil and a mating radiator, in the form of a spherical segment, to produce magnetically induced self-converging shock waves. Because the electromagnetic shock source does not deteriorate like the electrodes of the electrohydraulic shock wave lithotripters, use of electromagnetic shock wave lithotripters can provide time savings by not requiring the frequent replacement of the shock wave generating element. It has been shown, however, that electromagnetic shock wave lithotripters are often not as effective in stone comminution and patient outcome compared to first-generation electrohydraulic shock wave lithotripters, such as the original Dornier HM-3 lithotripters.
This deficiency is at least in part caused by the significant difference between the profile of pressure waveforms and lithotripter focal width (defined as extent of −6 dB contour of maximum peak positive pressure in the lithotripter focal plane) of electromagnetic and electrohydraulic systems. Specifically, whereas the pressure waveform produced by an electrohydraulic lithotripter consists of a leading compressive wave followed by a tensile wave, the pressure waveform produced by an electromagnetic lithotripter typically has an additional secondary compressive followed by an additional tensile wave of low amplitude at the end. The secondary compressive and tensile waves are caused by current oscillation produced during the energy discharge, an inherent feature of all electromagnetic shock wave lithotripters.
Methods and systems have been developed to modify and augment the operation of traditional shock wave lithotripters to at least partially account for the drawbacks inherent in both electrohydraulic and electromagnetic systems, but such systems can require additional components that add time and cost in setting up the lithotripter system. For instance, U.S. Pat. No. 5,224,468 to Grünewald et al. discloses an arrangement for generating focused shock waves from a combination shock wave source that can include a mixture of electrohydraulic, electromagnetic, and piezoelectric sources. Although the multiple shock wave sources can be set to define a time delay between pulses, such a complex system can require precise control and calibration of the multiple sources, and if an electrohydraulic source is included, all of the drawbacks to this technology remain. U.S. Pat. No. 4,972,826 to Koehler et al. describes an electromagnetic shock wave lithotripter in which one or more plates are inserted between the shock wave generator and the acoustic lens to modify the chronologically-varying pressure curve experienced at the focus of the shock wave lithotripter. These systems require additional components that must be properly aligned and adjusted within the system. Further, U.S. Pat. No. 4,664,111 to Reichenberger discloses a shock wave tube for generating time-staggered shock waves by means of a splitting device, such as a cone, for the fragmentation of concrements in vivo. Reichenberger discloses that the effects of the shock waves can be improved if they are so closely spaced in time that they overlap in their action on the concrement. The effects of shock wave induced cavitation are not disclosed by Reichenberger.
In addition, U.S. Pat. Nos. 5,582,578, 5,800,365, and 6,770,039 to Zhong et al., the disclosures of which are incorporated herein by reference in their entireties, each disclose methods for the comminution of concretions in vivo by controlling and concentrating cavitation energy utilizing two shock wave pulses separated by a short time delay. These methods are directed to using the second pulse to force the collapse of cavitation bubble clusters produced by the first pulse in such a way that the cavitation collapse energy is directed towards the target concretion and thereby reduce tissue injury caused by random collapse of cavitation bubbles. The multiple pulses of these methods, however, are created by modifications to a single shock-wave spark source or by the use of multiple shock-wave spark sources to create separate shock wave pulses. Furthermore, these methods generate the shock-waves using electrohydraulic systems rather than electromagnetic systems. The generation of multiple shock-wave pulses in electromagnetic systems would require a novel and entirely different physical principle to that used previously.